A gear reducer, also known as a speed reducer and gearbox, is a power transmission device that reduces the input rotational speed. Reducers help increase the torque of the motor and allow a receiving device to rotate under the impact of the created torque. These devices improve the efficiency of operations. For example, motors are usually designed to run at a specific RPM. Although this speed can be adjusted by using a variable frequency drive, in many cases it is more advantageous to use a specific set of gears to adjust the speed.

    Speed reducers connect a power source (usually an engine or a motor) to a driven load. It adjusts the torque/speed between the power source and the load by connecting the power source and the driven load with gears that have different reduction ratios. The reduction ratio is determined by the "teeth" on the gears. Meshing gears with different numbers of teeth changes the output speed/torque. Here is a video that demonstrates how the reduction ratio influences the output speed with Lego gears.